Charles Ellicott Commentary Proverbs 27:22

"Though thou shouldest bray a fool in a mortar with a pestle along with bruised grain, Yet will not his foolishness depart from him." — Proverbs 27:22 (ASV)

Though you should bray (i.e., pound) a fool (a self-willed, headstrong person) in a mortar among wheat with a pestle.—This would separate completely the husks from the wheat; but obstinacy has become a part of such a man’s nature, and cannot be got rid of even by such violent measures.